1. Field of the invention
The present invention concerns the thermoforming of parts from a synthetic material and more particularly an apparatus therefor.
2. Description of the prior art
There is already known a method of manufacturing small parts from synthetic material which consists in extruding a thin strip, storing it in spools and then mounting these spools one by one on the input to a fixed mold thermoforming machine.
In order to reconcile the continuous feeding of the strip with the stopping of the film material in line with the mold during the thermoforming operation, the film is formed into a loop which imparts some slack. During the thermoforming operation the strip is stopped on the downstream side of the loop whereas the material continues to arrive on the upstream side, the consequence of which is to increase the length of the loop. When the thermoforming operation is completed the strip resumes its progress on the downstream side of the loop and shortens it, and this process is repeated iteratively.
Given the mandatory presence of a loop this method is applicable only if the extruded film is very thin as must be the case, for example, to manufacture yogurt pots, drinking vessels, etc.
In practice there are other occasions for manufacturing objects by thermoforming although these must have a much greater thickness: boxes, automobile dashboards, sanitary devices, etc.
In this case it is not possible to carry out thermoforming continuously and it is necessary to use an intermediate stage which consists in manufacturing plates from granular synthetic material, these plates being then brought individually to a thermoforming mold.
A sequence of heating and cooling operations then results because the granular materials are first melted to obtain plates, these are then cooled for storage and transportation, and then the plates must be heated to the proper thermoforming temperature; finally, after thermoforming the object is again cooled. The consecutive thermal shocks resulting from these operations and the necessity for storing and transporting the plates are all costly and affect the quality of the products obtained.
Attempts have already been made to carry out thermoforming immediately after extruding a strip, without forming any intermediary products such as spools or plates.
For example:
French Patent No. A-1436-900 describes a vertical thermoforming machine provided with means for guiding an extruded film. This film is fed by rollers in the upper and lower parts of the machine but over all of the length of the film and not by its edges. Thermoforming is applied when the film is still hot, no means being provided for monitoring the temperature of the film, or for creating a cold area, or for subsequent heating of the material or, of course, for securing it laterally during heating.
French Patent No. A-1549-532 concerns a machine for thermoforming extruded film fed by rollers on the upstream and downstream sides of a thermoforming station but over all the length of the machine and not by its edges. The material delivered at constant temperature is deformed into an S shape at the output from the extruder (and is not left horizontal) by rollers which are designed to cool it to the thermoforming temperature. In this case there is no provision for rolling or creating a cold area or heating.
U.S. Pat. No. 3,562,859 describes a thermoforming machine operating in consecutive intermittent cycles and not continuously. A film is placed below a fixed heating station. When the film reaches the required temperature a mold is actuated against the lower side of the film towards the heating members, this mold remaining fixed throughout the molding operation and immediately thereafter is moved horizontally to a stationary cooling station. When this operation is completed the mold is drawn back to its retracted position and stripped from the formed film which remains below the fixed cooling station. The mold is then brought under the heating station for the next cycle, and so on.
This machine makes no provision for combined extrusion and rolling or, for a cold area between the extruder and a heating station, or for a heating station that moves with the mold, or for molding and heating during linear displacement with the film itself moving uniformly and continuously.
The reader should note that in order to be effective the heating station must be operative very close indeed to the film and that as a consequence of this molding in the upward direction can only be done over a very short distance, to create small objects of limited relief.
The present invention is intended to alleviate these disadvantages and to provide a continous thermoforming without any creation of intermediate products (plates or spools) since there is a direct transition from the granular material to a thermoformable strip.